Development of electrostatic images

ABSTRACT

Improved development of electrostatic images having features such as broad solid fill, continuous tone or low contrast is obtained by a two step process involving overdevelopment followed by selective cleaning. The overdevelopment is accomplished through the use of a development electrode that is either grounded or is biased to a polarity opposite to the polarity of the electrostatic image so as to enhance the deposition of toner onto the electrostatic image by creating an erected, unidirectional development field. This development field places toner uniformly on solid areas, proportionally on gray areas, in large amounts on low contrast image areas and also generally on background areas. The thus overdeveloped image is removed from the development field thereby establishing a bi-directional imaging clearing field that is directed toward desired image areas and away from background areas. A selective cleaning system such as a cascade of electrostatically attractive particles, is passed over the overdeveloped image in the presence of the image clearing field to selectively remove toner from background areas without equally removing toner from the image areas.

limited States Patent [191 Berlier et a1.

1 1 DEVELOPMENT OF ELECTROSTATIC lMAGES [75] Inventors: Richard A. Berlier; Robert T.

Ritchie, both of Lexington, Ky.

[73] Assignee: International Business Machines Corporation, Armonk, NY.

221 Filed: Dec. 21, 1970 211 Appl. No.: 100,229

[52] US. Cl 1l7/17.5, 96/1 SD, 118/637, 355/3, 355/17 [51] lint. Cl. G03g 13/08, G03g 15/08 [58] Field of Search l17/l7.5; 118/637; 96/1 SD, 1 R; 355/3; H17

[56] References Cited UNlTED STATES PATENTS 3,611,992 10/1971 Lyles l17/17.5 3,651,784 3/1972 Hewitt 1l7/l7.5 2,959,153 11/1960 Hider l17/17.5

3,592,675 7/1971 Cheng 118/637 3,146,687 9/1964 Donelson et al..... 117/l7.5 2,892,446 6/1959 Olden l17/l7.5 2,911,330 11/1959 C|ark.... ll7/17.5 3,589,895 6/1971 Ville ll7/l7.5 3,412,710 11/1968 Robinson 118/637 3,542,579 11/1970 Cundlach l17/17.5 2,735,304 1l/l955 Landrigan et al.... 117/1715 3,011,471 12/1961 Ulrich l17/l7.5 3,347,691 10/1967 Lyles 1l7/17.5 3,349,676 10/1967 Hudson ll7/17.5

[ 51 Aug. 28, 1973 Attorney-Hanifin and Jancin and E. Ronald Coffman [57] ABSTRACT Improved development of electrostatic images having features such as broad solid fill, continuous tone or low contrast is obtained by a two step process involving overdevelopment followed by selective cleaning. The overdevelopment is accomplished through the use of a development electrode that is either grounded or is biased to a polarity opposite to the polarity of the electrostatic image so as to enhance the deposition of toner onto the electrostatic image by creating an erected, uni-directional development field. This development field places toner uniformly on solid areas, proportionally on' gray areas, in large amounts on low contrast image areas and also generally on background areas. The thus overdeveloped image is removed from the development field thereby establishing a bi-directional imaging clearing field that is directed toward desired image areas and away from background areas. A selective cleaning system such as a cascade of electrostatically attractive particles, is passed over the overdeveloped image in the presence of the image clearing field to selectively remove toner from background areas without equally removing toner from the image areas.

5 Claims, 6 Drawing Figures Patented Aug. 28, 1973 2 Sheets-Sheet l FIG-I INVENTORS RlCHARD A. BERLIER ROBERT T. RITCHIE ATTORNEY Patented Aug. 28, 1973 3,754,962

Sheets-Sheet 2 FIG-3 DEVELOPMENT OF ELECTROSTATIC IMAGES DISCLOSURE OF THE INVENTION 1. Background of the Invention Development of a physical image from an electrostatic latent image has heretofore required a selection from among several development approaches each of which has its own characteristic advantages and disadvantages. One of the simpler development systems employs a cascade fiow of a mixture of toner and carrier particles over the electrostatic image. Localized fields, that define the electrostatic image, preferentially attract toner particles from the carrier onto localized sites. This method of development is particularly satisfactory for developing so-called line copy where images have a small relative width, are of generally equal density, and have a fairly high contrast with respect to surrounding background. Development of images having large solid areas or continuous tonal gradations, or low contrast portions are not satisfactorily developed with a simple cascade system.

Development electrodes have been employed to erect the electrostatic field in cascade systems and also in combination with so-called magnetic brush development systems and powder cloud development systems. The magnetic brush and powder cloud development systems however require a more complex and hence costly apparatus for their implementation. A development electrode used in conjunction with a cascade development system can be operated to enhance toner deposition, in which case a high level of background tone is experienced. On the other hand, it can be operated to suppress toner deposition by electrical biasing to the same polarity as the polarity of the electrostatic image to cause toner to prefer the development electrode to background areas. In this mode however the strength of the field available to deposit toner is reduced and the development of low contrast images is thus compromised.

It has thus been an object of-our invention to provide for improved development of electrostatic images having hard to develop characteristics such as large black areas, continuous tone images, and low'contrast images:

Another object of our invention has been to provide an improved electrostatic image development system employing a simple cascade technique capable of developing high quality, complex images;

A further object of our invention has been to optimize the implementation of a cascade development sys tem employing a development electrode and a conductive development carrier to provide an efficient, compact and relatively simple development system.

Briefly stated, the development system of our invention involves a first step of image overdevelopment followed by a step of image clearing or selective cleaning.

The overdevelopment step is preformed preferably by using a grounded development electrode to erect the electrostatic field presented by a latent electrostatic image on a photoconductive development surface. Toner is presented to the erected field by a cascading mixture of toner and carrier. The erected field effected by a grounded development electrode is uni-directional in the sense that the lines of electro-static force tend to move charge particles all in the same direction. The image however is characterized by variations in the strength of this uni-directional field as induced by the local charge variations that define the latent image. Accordingly, toner presented to the uni-directional field created by the development electrode will be driven by the electrostatic force to all parts of the charged surface. However, more toner will be driven to the more highly charged areas due to the increased strength of field. The image as thus overdeveloped will have a gray background, and due to-the absence of clear areas, the image will tend to ge generally gray and of low contrast.

The thus overdeveloped image is removed from the development electrode region thus re-establishing the original bi-directional electrostatic field that results from the localized surface variations in electrostatic charge. The lines of force of this bi-directional field tend to move a particle charged to one polarity from regions of low opposite polarity to regions of high opposite polarity. The bi-directional field thus creates a tendency for particles on background areas to be removed from the development surface and for particles on image areas to be retained on the development surface. While the forces involved are relatively low, they do define a tendency for movement, which when coupled with a mechanism for movement such as a further cascade of electrostatically attractive carrier particles, will cause selective cleaning by removal of those toner particles that tend to be removed by the bi-directional field without equal removal of toner particles that tend to be held by this bi-directional field.

The result of this two step process is a high quality, high contrast, physical image of the original latent image that includes full solid black areas, smooth continuous tone areas and adequately toned low contrast areas. A further benefit realized is a significant increase in the tolerance of the system to changes in toner concentration in the toner-carrier mix.

These and other features, objects and advantages of our development system will be further understood and appreciated by those skilled in the art from the following description of some specific illustrative preferred embodiments of our invention wherein reference is made to the accompanying drawings of which:

FIG. 1 is a schematic side cross-sectional view of an electrostatic photocopier showing the primary components thereof as related to our invention.

FIG. 2 is an enlarged cross-sectional view of a portion of the electrostatic photocopier shown in FIG. 1 showing details of a cascade development system constructed in accordance with our invention.

FIG. 3 is an enlarged graphic development of a portion of a development surface bearing an electrostatic latent image as is developable by our invention.

FIG. 4 is a cross-sectional explanatory view of a development surface bearing an image as shown in FIG. 3, illustrating the typical electrostatic fields relied on for operation of our invention.

FIG. 5 is a fragmentary schematic view of an electrostatic photocopier similar to that shown in FIG. 1 but illustrating a modification thereof.

FIG. 6 is a schematic view of a so-called treatedpaper type of electrostatic photocopier illustrating the application of our invention thereto.

Referring now more specifically to the drawings, in FIG. 1 there is shown a somewhat typical electrostatic image reproduction mechanism or photocopier l0 having a process drum 11 for supporting a photoconductive, image development surface or plate 12 for movement along a path 13 past successive processing stations. A charging station provided by a corona discharge device 14 effectively begins the process. At this station the plate 12 is uniformly charged to a high negative polarity. The plate 12 next moves an exposure station 15 where it receives the light and shadow image of an original document 16 which is projected in synchronism with the movement of the drum ll 1. The charge on plate 12 is selectively discharged in the light struck areas to form a latent image 60 of differential electrostatic charges (see FIGS. 3 and 4 hereinafter described in greater detail). The latent image pattern thus is of localized highly charged negative areas and less highly charged negative areas. The charge of these areas develops surface fields that can selectively attract toner particles that have been charged to the opposite or positive polarity The plate 12 is next passed through a development station 17 where electroscopic toner particles 2f!) (see FIG. 2) are presented to the fields defining the latent image 60 and are selectively attracted to the highly charged negative areas that were not discharged by light at exposure station 15. The pattern of accmulated particles 20 provides a physical image 30 corresponding to the latent image 60.

The thus created physical image is utilized in any of several known manners shown generally by the image utilization station 40. A common utilization involves transfer of the physical image 30 onto the surface of a separate image support surface such as paper 311. This transfer is accomplished by an image transfer corona 41 that electrostatically attracts the image 30 to the paper 31 from the plate 12. A fixing station 42 makes the transferred image permanent on the paper 33 and thus creates a relative permanent copy 32 of the original document 16. The plate 12 continues past cleaning station 18 where any remaining toner particles are removed prior to recharging by corona 14 for a further copy operation.

Alternatively it is known to have a similar electrophotographic system or photocopier (see FIG. ti) employing the photoconductive web, development surface or plate 12 that also forms the ultimate permanent copy. In photocopier 10', surface I2 is charged by corona l4, exposed at station then developed at development station 17. Instead of a transfer opera tion however, the physical image is utilized directly by the development surface 12 being passed directly into a fusing station 42 which fixes the developed physical image. A permanent copy 32'is thus formed.

IMPROVED DEVELOPMENT STATION Our invention provides an improved development station 17 shown in enlarged detail in FIG. 2. It is to be understood that the principles illustrated can be applied also to the development station 17 of the system shown in FIG. 6 as well as to other electrophotographic systems. Basic to the improved development station I7 is an overdevelopment substation 50 including a development electrode 51 and an image cleaning or clearing substation 52 immediately downstream of the overdevelopment substation 50. Also included in station 117 is appropriate mechanism for delivering a development mixture 21 of toner particles and carrier particles 22 to the substation 50 and 52.

A hopper or supply chamber 53 receives development mixture 21 from a bucket conveyor 54 Additional toner particles 20 are supplied from a replaceable cartridge 23 as toner is consumed. As known in the art, the surface tribo-electric characteristics of the carrier particles 22 and the toner particles 20 are selected such that frictional interaction therebetween, as occurs for example in the developer sump portion 54a (see FIG. 1) develops a positive charge on the toner particles 20 and a corresponding negative charge on the carrier particles 22. We prefer to use carrier made of spherical steel shot 24 within a size range between 300 and 600 microns diameter and coated with a thin layer 25 of pure polytetrafluorethylene resin. The positive charge on the toner particles 20 causes them to be attracted to the highly negatively charged areas of the electrostatic latent image 60 (see FIGS. 1, 3 and 4) and the negative charge on the carrier particles 22 causes them to attract toner particles 20 wherever they are not adequately attracted by part of the electrostatic latent image 60. The development electrode of substation 50 is provided by a conductive plate member 51 that is positioned in superimposed relation to the development plate 12 and is connected to ground by wire 51a. Electrode 51 is separated from development plate 12 by a spacing 55 and creates an erected uni-directional electrostatic field as more fully described hereinafter in connection with FIG. 4. A primary nozzle or discharge passage 56 delivers a flow 26 of the developer mixture 21 into the space 55 between the development electrode 51 and the photoconductor 12 to create a physical image 34 by the deposition of toner particles 20. The physical image 34 will be overdeveloped by having toner particles 20- on both desired image and background areas due to development electrode 51.

A portion of the development mixture 21 is diverted by the leading edge flow divider 51b of the development electrode 51 to bypass directly to a scavenging or secondary nozzle or passage 57 to be delivered as a cleaning or clearing agent 27 flowing at the image cleaning or clearing station 52. In this flow of clearing agent 27, negatively charged the carrier particles 22 in the mixture 21 directly attracts excess toner particles 20 from the plate 12. Also the flow of clearing agent 27 reactivates or agitates those carrier particles 22 flowing from the primary nozzle 56 as they emerge from the region of the development electrode 51 adjacent the plate 12. To optimize both actions, it has been found preferable for the scavenging nozzle 57 to be directed at a substantial angle of attack 57a such as an angle of about 17 or greater to the plate 12. Also it is preferable for the scavenging flow to carry approximately 95 percent of the combined flow through both nozzles 56 and 57. It is important that the fiow of clearing agent 27 be of relatively high velocity and we prefer a speed of between 40 and 50 inches per second for a plate 12 moving at 9.3 inches per second. The clearing action thus described attracts and removes undesired toner particles from background regions 33 of the overdeveloped physical image 34 to produce a high quality final physical image 30 with image regions 35 faithfully reproducing solid black, smooth continuous tones and low contrast features.

The detailed operation of our invention can better be understood from the explanatory views shown in FIGS. 3 and 4. In these figures the various components in FIG. 2 have retained the same number although they are shown in simplified form for ease of understanding. FIG. 3 shows the section of a development surface or plate 12 with the electrostatic pattern 60 being represented by shading of selected density corresponding to the relative levels of higher negative electrostatic charge. The image 60 comprises pattern features of large solid areas of high electrostatic charge such as 61, background areas of low electrostatic charge such as 62, intermediate or low contrast areas of moderate electrostatic charge 63 and even areas of continuously varying electrostatic charge 64. This typical pattern of electrostatic charge is also shown in the cross-sectional view of FIG. 4. It is pictorially represented by the amount of negative signs placed over the surface of the plate 12. The plate 12 includes a conductive backing plate 120 that is connected to ground through wire 12b. A conductive electrode 51 spaced above a portion of the plate 12 is also connected to ground through wire 51a.

As shown at the left of the electrode 51, field defining electrostatic lines of force 65 extend between areas of different electrostatic charge. These lines 65 can be considered as having a direction with respect to a charge particle of specific polarity. Specifically considering a positively charged particle with respect to the negative polarity of the image 60, the arrows or force lines 65 indicate direction that the particle will tend to move. Since these arrows are directed away from some areas of plate 12 and toward other areas, the field 66 they define is termed bi-directional. Thus a toner particle (see FIG. 2) charged positively and presented to the force lines 65 would tend to be deposited on the highly negative areas 61 of the latent image 60. The strength of the field represented by the lines of force 65 depends however not only upon the charge levels at the ends of the lines, but also the distance or length of the lines. Thus the highest electrostatic fields will occur at the edges or transition regions between adjacent areas of different charge. Conversely, relatively weak fields will exist in the middle portion 67 of a large black area such as 61.

In contrast with the bi-directional field on the left of FIG. 4, the presence of the grounded conductive electrode 51 creates an erected uni-directional electrostatic field 69 defined by lines of force 68. The force lines 68 extend from the lowest charge or ground a electrode 51 toward the negative charge of various degrees over the entire surface affected by the electrode 51. The force lines 68 are all directed toward the plate 12 both in the low negative background areas and the high negative image areas and are of the same length. Thus the force of field 69 tending to deposit a positively charged particle or plate 12 is substantially proportional to the localized charge pattern features of the latent image 60. Also, the presence of ground potential at one end of the field defining force lines 68 strength ens the entire field and provides a greater tendency to deposit toner on the low contrast areas 63 where only a moderate negative charge has been left by partial discharge at the exposure station 15.

The unique characteristic of these different and distinct fields, namely, the bi-directional field 66 as shown on either side of the electrode 51 and the erected unidirectional field 69 as shown in the region of the electrode 51, have been employed to produce the new results of our invention. The characteristic of the unidirectional field 69 is to deposit toner on plate 12 in proportion to the electrostatic image feature. Since plate 12 is never totally discharged, field will deposit toner in the back-ground areas 62 where no toner is desired. Having formed a physical image by depositing toner in this fashion, plate 12 is removed from the region of the electrode 51, to reestablish the bi-' directional field 66 with its tendency to repel positively charged particles from its background areas 62 and to attract positively charged particles to its image areas 61 as indicated by the arrows on force lines 65. While the strength of this field may be insufficient to spontaneously clear the background areas 62, it does assist in the selective cleaning by an appropriate clearing agent. Accordingly, when a clearing flow of carrier material 22 is passed over the over-developed physical image 34 as described in connection with FIG. 2, toner particles 20 are removed from background areas where the force lines 65 tend to cause removal, but are not equally removed from those areas such as the middle 67 of a large black section 61 where weak force lines 66 are tending to retain the toner particles.

Although our invention has been described specifically in connection with a cascade type development system which itself is particularly and conveniently simple, it will be understood by those skilled in the art that advantage can be gained by applying the principles of our inventive concepts to other development and cleaning systems. For example, the use of known magnetic brush techniques can be employed. Also, as shown in FIG. 5 the development electrode 51 can, if desired, be biased by an electrical source 58 preferably to a polarity opposite to the electrostatic pattern 60, in this case positive, to increase the overall deposition of toner onto the electrostatic pattern. Our invention cannot be practiced if the development electrode 51 is biased to the same polarity as the electrostatic image and of a magnitude equal or greater to that of the background areas of the electrostatic image, since to do so would not create the required uni-directional field, but instead would effect a lowered strength erected bi directional field that would be incapable of adequately developing low contrast or moderately charged areas such as area 63 shown in FIG. 4. It is of particular advantage to employ a positive biased development electrode where a development mixture of low development capacity is employed such as a conventional sand or glass carrier mixture. We prefer however to employ carrier particles 22 having a conductive core 24. The use of a conductive carrier itself acts to strengthen the various electrostatic fields, particularly in the presence of the development electrode. The carrier thus has a high capacity for laying down toner particles so as not to require the added complexity of a source of electrical bias on the development electrode.

In the event gravity forces do not sufiiciently accelerate the scavenging flow to a velocity substantially higher than the desired speed of the development surface, additional momentum can be achieved by the addition of a mechanical impeller. We have found the construction of a magnet within a high speed rotating non-magnetic, grit surfaced cylinder to be particularly efficient for adding momentum to magnetic carrier such as steel shot.

While our invention has been described in specific terms indicating the arrangement of a preferred illustrative embodiment and certain specific alternations, modifications and substitutions have been described, those skilled in the art will recognize that other details of construction and alternate embodiments can be employed to effect the inventive concepts that we have disclosed and that the subject matter sought to be patented is defined solely by the language of the appended claims.

We claim:

1. The method of developing a high quality physical image from an electrostatic pattern carried by a development surface, said pattern being characterized by having pattern features distinguished by electrostatic charge of varied magnitude of one polarity and including background regions, said method comprising the steps of:

creating an erected uni-directional electrostatic field adjacent said development surface,

cascading a mixture of carrier and said imaging particles over said development surface within the influence of said uni-directional field, said carrier and imaging particles being of differential tribo-electric characteristics so that their frictional interaction produces on said imaging particles the electrostatic charge of the polarity opposite to said one polarity whereby said particles are attracted differentially to said development surface in accordance with said pattern to form an overdeveloped physical image thereof,

thereafter establishing a bi-directional electrostatic field adjacent said development surface in accordance with said pattern, and cleaning said imaging particles from said background regions in accordance with said pattern to leave a high quality physical image thereof remaining by introducing a high velocity, high volume flow of relatively large particles of a material tribo-electrically related to said imaging particles so as to be attractive thereof, to said development surface within said hidirectional field and at a substantial angle of attack with respect thereto, and

thereafter utilizing said remaining high quality physical image to form a relatively permanent copy of said pattern.

2. The developing method as defined in claim 1 wherein said pattern comprises areas of at least three substantially different magnitudes of charge.

3. The developing method as defined in claim 1 wherein said utilizing step comprises transferring said remaining high quality physical image to a separate support surface.

4. The developing method as defined in claim 1 wherein said higher volume flow comprises at least of the combined flow of said cascading mixture and said high volume flow.

5. The developing method as defined in claim 1 wherein said relatively large particles are generally within the size range of 300 to 600 microns.

1 =l l =ll 

2. The developing method as defined in claim 1 wherein said pattern comprises areas of at least three substantially different magnitudes of charge.
 3. The developing method as defined in claim 1 wherein said utilizing step comprises transferring said remaining high quality physical image to a separate support surface.
 4. The developing method as defined in claim 1 wherein said higher volume flow comprises at least 95% of the combined flow of said cascading mixture and said high volume flow.
 5. The developing method as defined in claim 1 wherein said relatively large particles are generally within the size range of 300 to 600 microns. 